One of the small minnow mayflies I found yesterday in the Rivanna at Darden Towe Park (Charlottesville) was a new species for me: Iswaeon anoka. This was formerly known as Heterocloeon anoka, but in 2008, Guenther and McCafferty elevated Iswaeon to the status of genus (Steven Beaty, "The Ephemeroptera on North Carolina," p. 10).
The small minnow genus Iswaeon is described by Beaty in the following way:
"Genus Diagnosis: No procoxal osmobranchia; tibia narrow at base and distinctly widened medially to apically; claw with 2 rows of denticles with denticles in the first row about same length (except for first few); hind wing pads absent; two caudal filaments, median caudal filament of about 6 segments and slightly longer than abdominal segment 10." (Beaty, "The Ephemeroptera of North Carolina," p. 10)
There are three things in this description for which I cannot provide any photos. 1) The "procoxal osmobranchia" are the procoxal gills that are present on Heterocloeon curiosum nymphs (see the entry from yesterday): no sense in my taking a photo of something that doesn't exist. 2) The 2 rows of denticles on the claws can only be seen with 400X magnification. And 3) there are no hind wing pads, so again, I see no point in taking a picture of something that just isn't there.
But what is crucial in establishing Iswaeon identification -- according to Beaty -- is the tibia shape, and that the tibiae are narrow at the base and gradually widen, is easy to see from this microscope photo.
We can also see a close-up of the median caudal filament: it is indeed longer than abdominal segment 10.
Let's move to the species ID, and I'll go through Beaty's description section by section:
"I. anoka -- nymphs ~ 4 mm; a small, sometimes indistinct, dark dot at each tibial-femoral junction; a large, distinct dark spot laterally above the meso- and metanotal coxae." Those dots can be seen in this photo:
Beaty continues, "large median dark spots sometimes present on terga 2 and 6; midventral dark spots sometimes present on abdominal sterna 2-8 or 9." I'm not sure about the spots on terga 2 and 6, but there are certainly "midventral dark spots" on sterna 2-8. There are actually multiple spots visible on the sterna.
Beaty -- "dorsum of abdomen often with a pale median longitudinal stripe set against a darker background, sometimes reduced to small pale spots and interrupted on middle segments; caudal filaments with dark medial band."
The dark medial band on the caudal filaments (tails) is visible in the live photo at the top of the page. As for the pale longitudinal stripe on the dorsum "set against a darker background" -- if you enlarge the live photo, you can see a series of pale, longitudinal dash marks in the center of the terga. But the pale stripe against the dark background is very clear on segments 1-4 and 9-10 in this microscope shot.
I want to thank Steven Beaty for confirming my identification. I hope to return to the Rivanna tomorrow to find more of these nymphs and take better photos.
(For photographs of Heterocloeon anoka by Donald Chandler, go to: http://www.discoverlife.org/mp/20p?see=I_DSC233&res=640)